This invention relates to a vehicle air conditioning compressor having a swash plate to drive a plurality of pistons, each via a ball and a shoe arrangement. Such compressor have suffered in the past from a serious problem of lubrication, which is reflected in a large number of related patents and patent applications such as U.S. Pat. No. 3,759,058 dated Sept. 18, 1973. Nowadays the means of lubrication in such compressors is not necessarily a forced system (with a pump, etc.) but a system where the lubricant mixed with the cooling medium (e.g., Freon gas) is sprayed and brought to the spot necessitating lubrication by means of the centrifugal force, change in direction or speed of the medium gas flow, etc., effective for separating oil particles from conveying gas, however, there are not a few reasons to make the lubrication difficult.
Conventional shoes widely used in vehicle air conditioning compressors are made of Al-Si alloy casting which, for example, comprises Si 20 wt%, small amounts of Mn and Cu and the balance Al, or made of drawn rods of a Cu alloy which, for example, comprises Al 10 wt%, Fe 3.5 wt%, and impurities of less than 0.5 wt%.
Coventional shoes set forth above have been considered to be insufficient in abrasion resistance or shock resistance, because of a long term continuous operation (e.g, for 1200 hrs.) under a condition of oil shortage or difficult lubrication which results mainly from plane-to-plane contact between the shoe and the swash plate.
Therefore, it has been necessary frequently to exchange worn or broken shoes with new ones because of severe frictional wear or breakage due to impact, since the shoe is usually operated under a maximum pressure of about 90 kg/cm.sup.2 and a maximum sliding speed of about 23 m/sec. Reciprocating motion of the piston is accompanied by a repeated impact load due to pressure alternating from max. 90 kg/cm.sup.2 to min. 9 kg/cm.sup.2 at a rate of 50 cycles per second, when the rotating shaft is running at 3000 r.p.m., and the plane-to-plane contact between a swash plate and a shoe is generally accompanied by a condition making it hard to lubricate. hard to lubricate.